The septin cortex at the yeast mother–bud neck

Abstract

A specialized cortical domain is organized by the septins at the necks of budding yeast cells. Recent findings suggest that this domain serves as a diffusion barrier and also as a local cell-shape sensor. We review these findings along with what is known about the organization of the septin cortex and its regulation during the cell cycle.

Introduction

The ability to specialize specific domains of the cell cortex is crucial to the function of almost all cell types. Notable examples include the distinct apical and basolateral membrane domains of epithelial cells, the pre- and post-synaptic membranes of neurons, the leading edges of migrating cells, and the cleavage furrows of dividing cells. In the budding yeast Saccharomyces cerevisiae, a specialized cortical domain is established that separates the bud, which will grow into a new daughter cell, from the pre-existing mother cell. The neck between mother and bud will become the site of cytokinesis and septum formation at the end of the cell cycle.

Throughout the process of bud formation, the cell cortex at the neck is clearly differentiated from the cortex at other parts of the cell, both inside and outside the plasma membrane (Fig. 1). Outside, a chitin-rich ring (the eventual ‘bud scar’) forms in the cell wall on the mother side of the neck 1., 2., 3., 4.. Inside, a family of evolutionarily conserved proteins called septins [5] form a highly ordered cortical domain that can be seen as a set of ∼20 evenly-spaced striations spanning the neck by electron microscopy 6., 7.. Although the neck is unique to budding yeasts, septins are present widely, if not ubiquitously, in animal cells, in which they may also serve to organize cortical domains, including regions concerned with cytokinesis 8., 9., 10., 11., 12. or with regulated secretion 13., 14..

The septin-containing cortex in yeast is thought to act as a scaffold, recruiting many other proteins to the neck, where they can perform their varied functions (reviewed by Longtine et al. [5]). Well-established functions for the septin cortex include synthesis of the chitin ring and septum, bud-site selection and cytokinesis 15., 16., 17.. The ability of the septins to recruit chitin synthases, bud-site-selection landmarks, actomyosin ring components and other proteins to the neck is perhaps sufficient to account for their role in these processes. However, recent findings have documented novel roles for the septins that appear to entail more than simply recruiting other proteins to the neck. These studies highlight longstanding questions regarding how the septin cortex is organized and how it is regulated during the cell cycle. In this review, we discuss these recent findings with a focus on the regulation of septin organization and its implications for septin function.